An example of animal testing in the cosmetics industry - which is now banned in the UK and Europe

Scientists often use genetically modified animals to test how drugs work on certain diseases and deliberately breed "ill" animals with mutant genes.

To make sure that there will always be animals with the correct mutation available to test on, scientists tend to constantly breed animals - even when they aren't needed.

If researchers know that they might need seven animals later in the year, they will constantly breed them up until that point to ensure that there is supply.

Those that aren't used in the tests will be culled.

Mice are the most popular animal for testing, but there has been an increase in the use of dogs in Britain, latest figures show.

The Humane Society International says that surplus animals from continuous breeding programmes are not used for any specific testing or research purpose, "but are simply bred, caged and killed without any biomedical or scientific gain".

Behind the scenes at the Leicester University animal research facility

"They often live in very unnatural conditions and will see a certain amount of artificial light every day," the charity added.

"You often get stressed animals in these conditions, and you see mothers eating their own babies because it's so unnatural," Dr Lindsay Marshall, science communicators director at Humane Society International told the Sun Online.

But Tom Holder, Director of Speaking of Research, said the figures showed a positive change: "There was a 5 per cent drop in the number of procedures in 2016 when compared with 2015.

"97 per cent of these procedures were conducted on mice, rats, fish and birds.

"These experiments provide the scientific understanding we require to make medical and veterinary advances that improve the lives of us, our families and our pets."

Some genetic mutations cause catastrophic defects, such as mice born with shortened limbs, with no limbs, with the front of their head missing.

Mice are the most popular animals to test on as they breed easily and are small enough to manage with ease

If they survive birth, the animals could have significant developmental defects affecting their limbs, eyes, brain or hair.

They are predisposed to tumour formation and are often subjected to deliberate, repeated and ultimately fatal infections to assess the impact of the mutation, animal rights activists claim.

Any scientist that tests on animals must report the figures to the government and the Home Office is in charge of regulating research using them.

But the government was today slammed by Humane Society International and Cruelty Free International who claim they are not doing enough to stop what they describe as a "breed-kill cycle".

While many would argue that testing for medicines is imperative, whatever the means, they argue there is a different, fairer solution.

Scientists and private firms could freeze sperm or embryos to preserve them for later use.

The Medical Research Council’s frozen embryo and sperm archive in Harwell, and a number of animal supply companies, already offer this service, with facilities that can accommodate millions of embryos at a small fraction of the cost of maintaining a live colony.

Troy Seidle, Humane Society International's toxicology expert said: “Ultimately the best outcome for both science and animals is for our research effort to shift to cutting-edge humane alternatives, but in the meantime it’s galling to know that the number one opportunity to curb the explosive growth in UK animal procedures is being all but disregarded year after year."

The Home Office said that it is working hard to reduce animal waste but that medical testing is necessary

“Making sure that colonies of genetically-modified mice are managed efficiently is a very complex process. Researchers need mice of the correct age to be available at the start of their research project. As long as there is a long enough notice period, it may be possible to retrieve frozen embryos and implant them in surrogate mothers. But if the timing is not exactly right then the mice might not be ready in time, or be too old to be used.

Chris Magee, Head of Policy and Media at Understanding Animal Research said the activist's complaints should be taken with a pinch of salt.

He told the Sun Online: “Colony managers use a variety of ways to ensure that the numbers of animals used in research are minimised as far as possible, and they also work hard to give the mice as good a life as possible.

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"Mice are kept in enriched environments, with nesting materials and toys to play with. Lighting systems that mimic natural light with sunrise and sunset are also used and are designed to keep the animals’ stress levels as close to normal as possible."

A Home Office spokesman said: "The UK has one of the most comprehensive animal welfare systems in the world and we are completely committed to the proper regulation of the use of animals in scientific research.

"This research helps us to ensure that medicines are safe to use and to find treatments for cancer and other diseases, among a range of other benefits.

"Our legislation provides a rigorous regulatory system that ensures animal research and testing is carried out only where no practicable alternative exists and under controls which keep suffering to an absolute minimum."

Those interested in seeing lab animal's conditions can check here for a 360 degree tour.

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